The invention relates to a process for the regeneration of shaped bodies made of carbon and, in particular, graphite, which are used in the presence of elemental molten silicon, as well as to an apparatus for carrying out this process.
In high-temperature processes for technological applications, carbon and, in particular graphite, has been found to be a material with excellent properties. Even within the scope of the manufacture of high-purity elemental silicon as a base material for electronic components, solar cells or micro-mechanical components, carbon and especially graphite are used in the high-temperature steps. Carbon is used in this manufacture even though a direct contact between molten silicon and shaped carbon bodies has to be avoided as much as possible because of the formation of silicon carbide, which starts at high temperatures. For this reason, in processes, for example, requiring large amounts of molten silicon, e.g. in the pulling of silicon rods in crucibles according to the Czochralski process, or in the casting of silicon blocks, the melt is normally loaded in quartz crucibles which are externally surrounded by graphite crucibles. Although the graphite crucibles are therefore not in direct contact with the molten silicon, they are nevertheless highly stressed in these processes. Their frequent replacement is required due to the fact that their surfaces start to become brittle causing cracking and spalling, or their thermal conductivity and radiation characteristics change. Since these crucibles, often having large dimensions (typically about 400 mm diameter and about 300 mm height), have to be manufactured from massive blocks of graphite in most cases, their manufacture is expensive and complicated. Thus a frequent replacement of the crucible incurs significantly higher costs.
Accordingly it is an object of the present invention to provide a process for prolonging the useful life of shaped bodies made of carbon, and in particular graphite, which are used in the presence of elemental molten silicon.